Optical fibers used to carry many channels of information modulated light must often be joined on an end-to-end basis to couple the light from one fiber to another. Such jointings may be either permanent or provided in a manner which permits repeated connect and disconnect operations. Among the latter there exists a need for easy to operate jointing mechanisms, typically for use in the field by installers of optical fiber transmission lines. Field installation of cables, however, while not only difficult, increases the risk of fiber end contamination that reduces optical coupling efficiency.
One system which appears promising in such applications employs shape memory materials such as a nickel-titanium alloy. This material is capable of memorizing its shape when annealed at high temperatures, such as 500.degree. C. At room temperature the material may be deformed into another shape, but upon reheating to a predetermined temperature range will attempt to return to its shape at annealing even though restrained by a spring force. Upon subsequent cooling, the spring force returns the material to the deformed shape. This property has been utilized to provide optical fiber jointing by employing it in the fabrication of three V-shaped vanes which are set within a resilient collar defining a channel, at the apexes of the vanes. The channel can be selectively expanded by the application of heat to the vanes causing them to seek a less sharply angled shape memorized into the vanes at annealing. The expanded channel permits insertion of optical fibers from opposite ends which are subsequently captured within the channel when it contracts after the removal of heat. The channel design has an advantage in minimizing the risk of the fiber ends collecting debris during insertion, thus improving the chance for a high efficiency coupling.
While such apparatus appears useful for the coupling of optical fibers, the demands of the field environment where installers are required to make repeated splices and to uncouple and recouple fibers places demands upon such a jointing device which it is not by itself capable of meeting. One of the problems which field installers must face is the possibility of slight differences in optical fiber diameter between the two fibers being coupled. In this case, the collapsing channel will rigidly secure the end of the larger fiber but permit the smaller fiber to be less accurately seized at its end, permitting fiber misalignment that reduces the coupling between fibers. In addition, great care is required in the application of heat to the shape memory vanes to prevent over expansion of the channel to the extent that fibers slip between the sides of the vanes and further to prevent over stressing of the device from excessive heat. In addition, the channel diameter, even when expanded by heat, is exceedingly small, making field insertion of the fibers from each end a difficult, needle threading function. Even if one is successful in inserting the fibers from each end, there remains the problem of maintaining the fibers in end-to-end or near end-to-end contact during the cooling cycle of the shape memory material until it contracts the channel to capture the two fibers in proper end contacting relationship.